1. Field of the Invention
The present invention relates to a flux composition adaptable for controlling a slag flow in an electro-slag over-lay welding of a cylindrical vessel by using a strip electrode in down hand position particularly with an outer electro-magnetic field.
2. Description of the Prior Art
The inner surface of cylindrical vessels used for pressure vessels for atomic power generation and reactors for chemical plant is surfaced with a stainless steel and the like for providing corrosion resistance. The over-lay welding has been carried out by various processes but recently, the electro-slag over-lay welding with a strip electrode in down hand position in which the dilution by base metal is low, has generated great interest. This process includes flowing electric current through a molten slag from the strip electrode to generate Joule's heat and melting the electrode and the base metal due to this heat.
As the flux compositions applied thereto, a flux having a high content of CaF.sub.2 as disclosed in Japanese Patent Publication No. 53(1978)-29,657 and Japanese Patent Laid-open Specification No. 55(1980)-165,294 and a flux having a high content of TiO.sub.2 as disclosed in Japanese Patent Laid-open Specification No. 54(1979)-9,139 have been known.
The electro-slag over-lay welding process is an excellent one wherein the dilution by a base metal is low. However, when the process is applied for surfacing a cylindrical circumferential surface (particularly inner surface of the above described pressure vessel), the flowing state of the molten slag and metal varies at the edge portions and the central portion of the cylinder due to the means and place for connecting ground to the cylinder and to the influence of the magnetic field generated by a current supplying cable; or the molten metal flow varies in the vicinity of ground and the defects of under cutting and slag inclusion are apt to be caused. Particularly when a broad strip electrode is used for improving the surfacing efficiency, the electric current becomes large and therefore the above described tendency becomes noticeable and therefore, even if the conditions under which the good bead can be experimentally obtained by means of a broad electrode, are adopted as such, such conditions often cannot be applied as the satisfactory electro-slag over-lay welding process in practice.
Furthermore, these cylindrical bodies are made by bend-working a steel sheet, so that it is impossible in view of the working precision to obtain a true cylinder. When this distorted cylinder is subjected to the over-lay welding while being rotated on a turning roller, the position where a strip electrode for the electro-slag over-lay welding is set, influences or effects the welding, so that careful consideration should be given to the variation of the position.
Japanese Patent Laid-open Specification No. 55(1980)-136,566 (U.S. Pat. No. 4,309,587) has disclosed that an outer electro-magnetic field is applied to a welding current to generate Lorentz's force, whereby the flow of the molten slag is controlled as shown in FIG. 2. If the flow of the molten slag and metal is controlled with an outer electro-magnetic field as shown in the above U.S. Pat. no. 4,309,587, the variation of the flow due to the external factor is effectively restrained and the excellent surfaced bead having no under cutting nor slag inclusion can be easily obtained even by using a broad strip electrode.